1. Field of Invention
The invention is related to an image forming apparatus for forming an image on a printing medium or an image reading device for reading images of an original in a facsimile device or a copying device. Specifically, the invention is related to the tension adjustable mechanism of a device that reciprocatingly moves the carriage on which a print head and/or a scanner is mounted.
2. Description of Related Art
There are printing units of a facsimile device or a copying device for forming images on a printing medium, having a structure such that the carriage on which the print head is mounted, is connected to a wire or a timing belt wound around a driving pulley mounted to the output shaft of the stepping motor (driving motor), and a following pulley, so that the carriage reciprocatingly moves in a perpendicular direction to the printing medium transferring direction.
When the stepping motor starts and stops or the carriage changes its movement, quite a large tension occurs in the endless wire or timing belt from the inertia of the carriage. Therefore, if the pulley is fixed to the frame (chassis frame) and the rigidity of the supporting portion of the pulley is not large, vibration occurs and the hunting phenomenon occurs when the carriage moves. Therefore, the printing (formed image) quality becomes bad because the formed images are out of position when the carriage moves reciprocatingly.
When a timing belt is used and the belt becomes slack due to age, the tension power set when the belt was new is decreased drastically and the engagement of the gears becomes worse between the timing belt and the following pulley. Thus, the durability is decreased.
To solve the above-described problems, Japanese Unexamined Patent Publication No. 58-3891, Japanese Unexamined Patent Publication No. 63-176649 and Japanese Unexamined Patent Publication No. 3-146381 disclose the following structure. The driving pulley is fixed to the frame and the bracket supporting the following pulley is supported movably parallel to the direction of the tension added to the endless belt. A tension spring, a compression spring, or the like, for adding tension, is mounted to the bracket.
However, in the above-described prior art, when the heavy carriage mounting the print head starts to move suddenly from its stationary state (the starting time of the printing operation), the force of inertia is generated. Therefore, the driving pulley and the following pulley are easily moved against the force of the spring in a direction such that the distance between the axes of the pulleys becomes short, and the tension applied by the spring is extremely decreased momentarily and the timing for movement of the carriage is thrown off. In particular, when a timing belt is used, the printing quality becomes extremely bad because the gear of the following pulley and the engaging surface of the timing belt, do not engage properly.
One aspect of the invention is to provide a printing apparatus having no decrease of durability and no decrease of printing quality. To achieve this aspect, an image recording/reading apparatus of the invention records/reads an image by reciprocatingly moving a carriage on which a print head and/or a scanner is mounted. The image recording/reading apparatus includes a first pulley arranged fixedly and rotatably in the image recording/reading apparatus, a driving source for driving to rotate the first pulley, a bracket arranged movably relative to the image recording/reading apparatus, a second pulley rotatably supported to the bracket, a belt provided over the first pulley and the second pulley and connected to the carriage, a pressing member arranged between the image recording/reading apparatus and the bracket for applying tension to the belt, and a restricting device for restricting movement of the bracket in a direction that the belt is loosened.
In the image recording/reading apparatus, the belt for reciprocatingly moving the carriage on which the print head and/or scanner is mounted, is provided over the first pulley arranged fixedly in the apparatus and the second pulley rotatably supported by the bracket arranged movably relative to the apparatus. The pressing member is provided between the apparatus and the bracket for applying tension to the belt. The restricting device restricts the movement of the bracket in the direction that the belt is loosened. Therefore, when a large tension is suddenly generated to the belt, for example when the carriage starts to move, the movement of the bracket is restricted so that the belt is not loosened and the movement of the carriage is stable from the start of the operation. When the belt is a timing belt, the improper engagement of the gear of the second pulley and the engaging surface of the timing belt can be prevented, and therefore, the reduction in printing quality can be prevented.
Further objects, details and advantages of the invention will be apparent from the following detailed description, when read in conjunction with the drawings.